This invention relates to a method and apparatus for encoding an n-bit information word (n.gtoreq.2) into an m-bit NRZI code word (m&gt;n), wherein the DC component of successive NRZI code words is minimized.
In many applications, a digital signal formed of a number of binary "1"s and "0"s is represented in a nonreturn-to-zero (NRZ) format. Such NRZ digital signals may be transmitted from one location to another directly, and may even be recorded on a magnetic medium without further modulation. To reduce errors that may be caused by dropouts, distortions, and the like of the NRZ signal, the digital information may be additionally encoded in any one of a number of error-correcting codes.
Recently, digital recording techniques have been used for the high fidelity recording of video signals. In a digital video tape recorder (DVTR), the video signal, which normally is in analog form, is converted to a corresponding digital video signal, and the digital video signal may be encoded in an error-correcting code and then recorded in NRZ format. In one DVTR, the digital video signals are formed as 8-bit signals and a number of successive 8-bit signals are recorded. For synchronizing, error-detecting and error-correcting purposes, the 8-bit digital video signal also is interspersed with digital synchronizing signals, patterns, and the like.
In magnetic recording, as well as in various signal transmission systems, a DC signal is neither recorded nor reproduced accurately. Hence, when digital signals are provided in NRZ format, the magnetic recording of such signals generally is constrained to the recording of the bit transitions therein. That is, the change-over in the digital signal between a binary "0" and a binary "1" is recorded. However, the constant positive or negative DC level of the binary "1" and "0" is lost. Therefore, distortion may be introduced into the recorded NRZ signal, and additional distortion is present when the recorded signal is reproduced.
Because of the foregoing disadvantages in magnetic recording and in some signal transmission systems, it is desirable to encode an information word, such as an 8-bit digital video signal, in a form which avoids or compensates for many of such disadvantages. For example, it is desirable to minimize distortion that may be present if a long run-length of binary "0"s and "1"s is present. Although there may be no practical control over the run-length of the original information words, such words may be encoded so as to be represented by code words that avoid long run-lengths. Thus, an information word which consists of all "0"s or all "1"s may be represented by a code word having a number of bit changeovers therein. This means that such code words will be formed of a larger number of bits (e.g. m bits) than the number of bits (e.g. n bits) which constitute the information word.
In addition to avoiding long run-lengths, another desirable characteristic of some encoding techniques that have been proposed is to minimize the effective DC component of successive code words. For example, in the NRZ format, if a binary "1" is represented by a DC level of +1 volt and a binary "0" is represented by a DC level of -1 volt, then the overall DC component, or disparity, of a digital word is obtained by summing the DC levels which represent the bits included in that word. If the disparity of a digital word is positive, then the number of binary "1"s included therein exceeds the number of "0"s in that word. Conversely, if the disparity is negative, then the number of binary "1"s is less than the number of binary "0"s in that word. Over a period of time, the summation of the disparities of successive digital words results in a positive, negative or zero value that is a direct representation of the overall DC component of such words. This accumulation of disparities is known as the digital sum variation (DSV). It is appreciated that if each digital word is formed of an odd number of bits, then each digital word will exhibit non-zero disparity. The accumulation of such disparities, that is, the digital sum variation, if not controlled, may become excessively positive or negative. Thus, the DC component of the digital signal formed of successive digital words may become excessively positive or negative.
A large DC component in digital signals is to be avoided in those systems, such as magnetic recording, wherein a DC component is not accurately maintained. This is because large DC components result in distortion, as mentioned above, thereby imparting errors into the recovered digital signal. One technique for maintaining a low DC component in the DC signal is known as low disparity encoding. In this technique, an input digital information word is converted to a code word having a substantially greater number of bits than the information word and, moreover, the code word being formed of an even number of bits. A known low disparity code is the so-called (4, 6; 0) code, wherein a 4-bit information word is represented by a 6-bit information word, each information word exhibiting zero disparity. It is recognized that the (4, 6; 0) code is easily attainable because sixteen different information words may be represented by a 4-bit word; and in a 6-bit code word there are twenty individual words which exhibit zero disparity. Thus, there are more than enough 6-bit zero disparity code words to represent the 4-bit information words. Also, the run-lengths of the 6-bit code words are relatively small.
However, in the (4, 6; 0) low disparity code, a large number of bits of the code word are provided merely to make sure that the disparity of that word is maintained at zero. Such code word bits are not needed to represent useful information and, therefore, they are redundant. These redundant bits, when recorded, occupy an area that otherwise could be used for information. Hence, a relatively high recording density is needed when recording low disparity code signals of the (4, 6; 0) code. Furthermore, since a large number of redundant bits are recorded with this low disparity code, the effective "detecting window", that is, the interval which is available for detecting each bit, is reduced from that which otherwise could be used if the original information word is recorded. For example, in the (4, 6; 0) code, a detecting window whose effective interval is equal to the four information bits now must be used to detect the six code bits. Consequently, there is a greater possibility of introducing error into the reproduced low disparity code word.
Yet another disadvantage of low disparity codes having high redundancy, such as the (4, 6; 0) code, is that if a read only memory (ROM) is used to convert an information word into a code word, the ROM must have a very high storage capacity.
Many of the foregoing disadvantages and difficulties have been overcome by the encoding technique described in copending application Ser. No. 201,781 now abandoned, filed Oct. 29, 1980. The encoding technique described in that application contemplates the conversion of n-bit information words into m-bit code words in the NRZ format. While the encoding technique is successful, thus enabling digital signals to be recorded and reproduced accurately, the use of the NRZ format requires strict control over the polarities of the various components included in the recording or signal processing system. If the polarity of the windings on the recording/reproducing transducer, or head, or the polarity of the recording or playback amplifiers is reversed, then a signal which had been recorded as a binary "1" in the NRZ format may be reproduced as a binary "0". Likewise, if signals had been recorded on a magnetic medium by one recording system and are reproduced by another having different polarity, those signals which originally had been recorded as binary "1"s will be reproduced as binary "0"s, and vice versa. This is because, in the NRZ format, it is the direction of transition from one level to the next that is representative of the binary signal. Thus, a positive transition represents a changeover from a binary "0" to a binary "1" and a negative transition represents a changeover from a binary "1" to a binary "0". In the event of any reversals in the polarity of the magnetic medium, the windings of the record/reproduce transducer, the amplifier circuitry, and the like, the sensed polarity of the transition between binary "1" and "0" likewise will be reversed so as to make the reproduced NRZ signal erroneous.
Because of the foregoing critical dependency of the NRZ format on polarity, the assembly and repair of, for example, a recording or reproducing system must be carried out with great care. Furthermore, this polarity dependency of the NRZ format limits the improvements, or retrofitting, of the recording apparatus.
The foregoing disadvantages of the NRZ format are substantially minimized, or avoided, by modulating the digital signal in nonreturn-to-zero inverted (NRZI) format. As is known, in the NRZI format, a binary "1" is represented by a transition of either positive or negative polarity, and a binary "0" is represented by the absence of a transition. Since it is the transition itself and not the polarity thereof that is representative of the binary signals, the aforementioned polarity dependency and defects of the NRZ format are avoided by using NRZI modulation. However, if n-bit information words merely are modulated in NRZI format and recorded directly without any additional encoding, the above-described disadvantages associated with long run lengths and DC components may result. Consequently, it is advantageous to encode an information word into a low disparity code word, and then modulate that low disparity code word in NRZI format prior to recording, transmission or processing.